Multi-mode laser diodes (MMLDs) are frequently used in electro-optical devices. An MMLD is a laser diode that supports at least two stable modes, though these modes are not necessarily supported at the same time. Typical operation of the laser diode may operate by injection of a current into the device, which causes the coupled lasers to alternate between modes. In many embodiments it is desirable to use MMLDs as optical oscillators, in which case the MMLD include electrical signal inputs along the longitudinal axis of the device. These electrical signals cause perturbations in the mode structure of the MMLD, activating additional modes of the MMLD. The outputs of the MMLD are similarly located along the longitudinal axis of the device.
Though these devices are effective in many situations, some problems do exist. One of the most significant limitations is the cross-talk between the input signals and the output signals. As is apparent to those of skill in the art, cross-talk is the phenomenon where signals from one channel cross over into another channel and vice versa in a multi-channel environment. This severely limits the accuracy of the fabricated device and prevents the use of such devices for a number of purposes. Current devices also encounter problems with discrimination between output modes. In most logic devices the output waveguides are closely spaced. This makes discrimination of the modes difficult, resulting in the operation of unwanted modes. MMLDs present the result of the logical operation as the presence or absence of a signal in a specific mode. If the modes can not be accurately discriminated, the logical results can not be cascaded to further logic gates. Finally, MMLDs are generally large in terms of other opto-electronic devices. The length is necessary to allow mode interaction given the typical construction, which includes parallel waveguides and on-axis input signals.
U.S. Pat. No. 4,252,403, entitled “COUPLER FOR GRADED INDEX FIBER,” discloses a basic construction for propagation of multiple modes between two fibers. Specifically, this patent discloses a concentric core fiber that is coupled to a graded index fiber by precisely aligning the concentric core fiber with the graded index fiber. Low order modes propagating in the concentric core fiber are coupled to the central region of the graded index fiber and high order modes of the concentric core fiber are coupled to the inner core of the graded index fiber. The present patent does not use this method for propagation of signals. U.S. Pat. No. 4,252,403 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,363,463, entitled “REMOTE SENSING OF FIBER OPTIC VARIABLES WITH FIBER OPTIC SYSTEMS,” discloses a variety of fiber optic systems. One such system includes structures for diverting temperature and force information from the optical fiber of the system for measurement, and for reintroducing information into the system at any one of a number of points along the side of the optical fiber. The optical fiber is appropriately prepared along its side to have information reintroduced at certain points. The present invention does not operate according to these principles. U.S. Pat. No. 5,363,463 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,764,681, entitled “DIRECTIONAL CONTROL METHOD AND APPARATUS FOR RING LASER,” discloses a method for controlling direction of coupled lasers. Specifically, an asymmetric feedback structure is used to actively or passively control lasing of the system. Active control, which permits directional control of the laser, is achieved through the use of a dielectric stack. Passive control operates through introduction of asymmetry into the path of the laser, preferably by use of a diode to produce cross-coupling of two modes of a laser. The present invention does not operate according to these principles. U.S. Pat. No. 5,764,681 is hereby incorporated by reference into the specification of the present invention.
“A New Design for Ultracompact Multimode Interference-Based 2X2 Couplers,” David S. Levy, et al., IEEE Photonics Technology Letters, discloses a splitter device that takes a single input and splits the signal into two outputs. The single input is introduced along the axis of the splitter. The structure includes two tapered structures, the tapered ends of the structures being joined in the centers. Both parabolically and linearly tapered devices are proposed. The device includes both two inputs and two outputs, the signal being propagated along the axis of one of the inputs. The present invention does not operate in this manner. This disclosure is hereby incorporated by reference into the specification of the present invention.
“Analysis of the Dynamic Behavior and Short-Pulse Modulation Scheme for Laterally Coupled Diode Lasers,” Horatio Lamela, et al., IEEE Journal on Selected Topics in Quantum Electronics, discloses a structure having two parallel lasers that allows mode coupling between the lasers. Specifically, the two parallel lasers, given sufficient length, are spaced closely enough to allow switching of power from one laser to the other. To achieve this, the lasers must both be longer than is desirable for most mode switching applications and must be very closely spaced, which makes the modes difficult to isolate. This structure is not the same as that of the present invention. This disclosure is hereby incorporated by reference into the specification of the present invention.
“Multimode Interference Bistable Laser Diode,” Mitsui Takenaka, et al., IEEE Photonics Technology Letters, discloses a bistable laser structure that switches output modes when an appropriate optical input signal is injected. Specifically, a signal is input along the axis of one of two waveguide inputs. The two waveguides are closely spaced to allow coupling into modes within the bistable laser. The bistable laser must be of sufficient length to allow mode interaction with the injected input signals. As was explained previously, this length is generally greater than is desirable for most multimode applications. Additionally, the close spacing of the output waveguides makes mode discrimination difficult and the on-axis input signal injection is prone to unintended signal scattering back into the input waveguides. The present invention does not operate in this manner. This disclosure is hereby incorporated by reference into the specification of the present invention.
Prior art multimode laser diode photonic logic devices have significant limitations in the electro-optical arts. Specifically, to allow interaction between the modes and input signals, most devices must be long. Because a primary goal of electro-optical device designers is to obtain the most compact device possible, this is a significant limitation. Further, most prior art devices operate by closely spacing the output waveguide structures, which results in an inability to accurately discriminate between modes of the output signal. It is therefore desirable in the art to have a compact multimode laser diode photonic logic device that allows accurate discrimination between the modes of the output signal. It is further desirable in the art to have a multimode laser diode photonic logic device that can accept an input signal off-axis with respect to the output signal to prevent unintended signal scattering back into the input waveguides.